Polytypism of Cronstedtite from Ouedi Beht, El Hammam, Morocco

• Published in: Clays and clay minerals 2021-12, Vol.69 (6), p.702-734
• Main Authors: Hybler, Jiří, Dolníček, Zdeněk, Sejkora, Jiří, Števko, Martin
• Format: Article
• Language: English
• Subjects: Aggregates; Biogeosciences; Chemical composition; Crystals; Diffraction patterns; Earth and Environmental Science; Earth Sciences; Electron probe; Electron probe microanalysis; Geochemistry; Magnesium; Manganese; Medicinal Chemistry; Mineralogy; Nanoscale Science and Technology; Polytypes; Rotation; Sequences; Silicates; Silicon; Single crystals; Soil Science & Conservation; X-ray diffraction
• ISSN: 0009-8604; 1552-8367
• DOI: 10.1007/s42860-021-00157-2

The present study deals with accurate identification of polytypes, twins, and allotwins – oriented crystal associations of more than one polytype. The trioctahedral 1:1 layered silicate cronstedtite was studied using single-crystal X-ray diffraction data collected with a four-circle diffractometer equipped with a CCD detector. The sample from the skarn occurrence, Ouedi Beht, El Hammam, Morocco, was explored. It contains cronstedtite in fibrous massive aggregates in the central part, and euhedral crystals in surrounding veinlets and druses. The reciprocal space (RS) sections created by the diffractometer software and presented here were used to determine the OD (ordered-disordered) subfamilies (Bailey’s group A, B, C, D) and to identify polytypes. The chemical compositions of some crystals were determined thereafter by electron probe microanalysis (EPMA-WDS). Some crystals studied are more or less complicated allotwins. Polytypes were thus separated by cleaving crystals into smaller parts in many cases. All polytypes found belong to subfamilies A or D. The following polytypes of the subfamily A were identified: 2 M 1 ( a = 5.49, b = 9.51, c = 14.40 Å, β = 97.30°, space group Cc ), 1 M ( a = 5.51, b = 9.54, c = 7.33 Å, β = 104.5°, Cm ), 3 T ( a = 5.51, c = 21.32 Å, P 3 1 ), 6 T 2 ( a = 5.50, c = 42.60 Å, P 3 1 ). 2 M 1 and 3 T were present as isolated crystals or separated by cleaving, otherwise all these polytypes are parts of allotwins. The 2 M 1 polytype is sometimes twinned by reticular pseudo merohedry with twin index n = 3 and 120° rotation about the c hex axis as the twin operation. Allotwins of 1 M + twinned 2 M 1 polytypes are also present. Another kind of twinning, with rotation by (2 n +1)×60° about c hex is rare. The subfamily D is represented mostly by 2 H 1 and 2 H 2 polytypes, a = 5.50, c = 14.25 Å, space groups P 6 3 cm (2 H 1 ), P 6 3 (2 H 2 ). In addition, several six-layer ( a = 5.49, c = 42.80 Å), mostly non-MDO polytypes were separated from allotwins by cleaving. In order to identify them, 24 possible stacking sequences were modeled, diffraction patterns calculated, graphical identification diagrams constructed, and comparisons made with actual RS sections. This simulation revealed that five pairs of sequences provided identical diffraction patterns. Polytypes actually found correspond to the following sequences: 1 (6 T 1 ), 5 (proposed Ramsdell’s symbol 6 T 3 ), 8+10 (6 T 5 ), 11+12 (6 T 4 ), 24 (6 T 6 , trigonal polytypes, space group